User Guide

Memory Model 13
24592—Rev. 3.15—November 2009 AMD64 Technology
Figure 2-4. Legacy-Mode Memory Management
The memory-management functions differ, depending on the submode, as follows:
• Protected Mode—Protected mode supports 16-bit and 32-bit programs with table-based memory
segmentation, paging, and privilege-checking. The segmentation function takes 32-bit effective
addresses and 16-bit segment selectors and produces 32-bit linear addresses into one of 16K
memory segments, each of which can be up to 4GB in size. Paging is optional. The 32-bit physical
addresses are either produced by the paging function or the linear addresses are used without
modification as physical addresses.
• Virtual-8086 Mode—Virtual-8086 mode supports 16-bit programs running as tasks under
protected mode. 20-bit linear addresses are formed in the same way as in real mode, but they can
optionally be translated through the paging function to form 32-bit physical addresses that access
up to 4GB of memory space.
• Real Mode—Real mode supports 16-bit programs using register-based shift-and-add
segmentation, but it does not support paging. Sixteen-bit effective addresses are zero-extended and
added to a 16-bit segment-base address that is left-shifted four bits, producing a 20-bit linear
address. The linear address is zero-extended to a 32-bit physical address that can access up to 1MB
of memory space.
513-185.eps
031
Protected Mode
031
Paging
Physical Address (PA)
Linear Address
Virtual-8086 Mode
019
031
Paging
Linear Address
Physical Address (PA)
Real Mode
019
19 031
Linear Address
0
PA
Segmentation
031015
Effective Address (EA)Selector
015
EA
Segmentation
015
Selector
015
EA
Segmentation
015
Selector